By combining the activity of cholesterol oxidase and cholesterol hydrolase with a polarographic anode sensitive to hydrogen peroxide, it is possible to measure cholesterol directly in blood serum. Further the electrode and cuvet can be designed so that samples as small as 20 microliters can be analyzed in less than one minute. The present research is to investigate the electrochemistry and kinetics of the main reactions so as to perfect a method for the analysis of blood and tissues which can be widely used at low cost. Means of immobilizing the enzyme on membranes are being explored. A further object is to develop new cholesterol standards and to examine the distribution of cholesterol in cells and tissues. Attempts are being made to devise an electrode system capable of rapidly determining cholesterol fractions bound to the high and low density lipoproteins.